This invention relates to chemical process equipment in which a liquid is contacted with a counterflow of gas. This may be for a variety of purposes such as stripping a component from the liquid stream or absorbing a component into a liquid stream. More generically this invention relates to equipment designed to facilitate mass and/or heat transfer between phases.
The type of equipment to which this invention specifically relates employs cross-flow fractionation trays connected by downcomers. In such equipment a tower is provided with a plurality of fractionation trays arranged generally horizontally within the tower. Each tower has a perforated deck and at least one channel, called a downcomer, in which a liquid flowing over the deck may be collected and channeled to the tray below. In use a gas or vapor is introduced at the base of the tower and passes upwards through the perforations in the decks of the fractionation trays. Meanwhile a liquid is introduced at the top of the tower and percolates downward passing over the fractionation trays and down the downcomers to the tray below. Liquid exits the downcomers in a typical design either through an open bottom and/or the downcomer front area, (that is the side facing towards the center of the tray). The deck area available for perforation lies between the area below the downcomer from the tray above and the top of the downcomer to the tray below. This perforated area is known as the "bubbling area". The maximum vapor/liquid capacity of a tray increases with the increasing size of the bubbling area. One method of increasing the size of the bubbling area is to cut the downcomers short and perforate the area under the downcomer, (the "under-downflow area"). In such arrangements the liquid from the foreshortened downcomer discharges on to a perforated under-downflow area.
In some cases the downcomer may have a bottom pan where liquid flows around and out through slots in the bottom. The flow of liquid from the downcomer directly on to the perforations in the under-downflow area can be a problem however. In some designs there is provision for a raised perforated area under the downcomer with deflectors preventing direct contact between vapor and liquid, or a shelf over the perforated portion of the under-downcomer area.
The bubbling area of a tray can use a number of devices for passage of gas from below the tray for contact with the liquid flowing across the tray. These could be plain holes as in "sieve trays", or holes with variable orifice devices known as "valve trays", or gas chimneys fitted with inverted bell-shaped or rectangular caps known as "bubble cap trays".
According to an idealized process design, the liquid should be prevented from passing through the perforations in the decks of the fractionation trays by the pressure of gas passing through the perforations in the upward direction. This is a finely balanced process since, if the pressure is too great, the gas will have a shorter transit time within the tower and less efficient contact with the down-flowing liquid. The high gas velocity may also cause liquid droplets to be carried up to the tray above, thereby reducing the separation efficiency as a result of back-mixing. On the other hand if the gas flow rate is too low the liquid will penetrate through the perforations in the tray decks, (known as "weeping"), and short-circuit the flow patterns which are intended to maximize the extent and efficiency of liquid/gas contacts.
Thus, in summary, the gas flow should be slow enough to permit efficient contact with the liquid flow but fast enough to minimize weeping. While a pressure differential between the space above a fractionation tray and the space below is necessary, if this differential is too great gas flow will be accelerated as it passes through the perforations and the efficient bubbling contact will be lost.
Weeping is however often a problem when the liquid flow rate is particularly heavy in a local perforated area, and particularly in the under-downcomer area, and the present invention provides a fractionation tray design that ensures that the danger of weeping is minimized.
European Patent Application No. 0 266 458 discloses a tray construction in which valves similar to those on the main active area of the tray are placed beneath the under-downflow area.
In U.S. Pat. No. 5,106,556 the under-downflow area is provided with a raised platform with louvred vents through which the vapor can pass while being directed away from the under-downflow area.
It is an object of the present invention to provide a structure with a very high capacity in terms of throughput with very high efficiency from the viewpoint of the degree of separation that is attained.
The present invention provides a tray with maximized capacity by using the under-downcomer area as part of the bubbling area and selecting a novel device to prevent weeping of liquid through the tray to the tray below while maximizing opportunities for vapor liquid contact.